1. Field of the Invention
The present invention relates to a laser processing apparatus for processing a workpiece such as a semiconductor workpiece by applying a processing laser beam.
2. Description of the Related Art
In a semiconductor device fabrication process, the front side of a substantially disk-shaped semiconductor workpiece is formed with a plurality of crossing streets (division lines) to thereby define a plurality of partitioned regions arranged like a matrix, and a plurality of circuits such as ICs and LSIs are formed in these partitioned regions. The semiconductor workpiece is cut along these streets to divide the partitioned regions where the plural circuits are respectively formed, thus fabricating a plurality of individual semiconductor chips. Cutting of the semiconductor workpiece along the streets is usually performed by using a cutting apparatus called a dicer. Another cutting method using a laser beam is also known.
For example, as a method of dividing a workpiece such as an optical device wafer along the streets, a method of forming a laser processed groove is known. In this method, a pulsed laser beam having an absorption wavelength to the workpiece is applied to the workpiece along each street to thereby form a laser processed groove along each street. The workpiece is next broken along each laser processed groove by applying an external force (see Japanese Patent Laid-Open No. Hei 10-305420, for example). Also known is another laser processing method using a pulsed laser beam having a transmission wavelength to a workpiece, wherein the pulsed laser beam is applied to the workpiece so as to be focused inside of the workpiece. In a workpiece dividing method using this laser processing method, the pulsed laser beam having a transmission wavelength to the workpiece is applied to the workpiece from one side thereof so as to be focused inside of the workpiece along each street, thereby continuously forming a modified layer inside the workpiece along each street. By forming the modified layer inside the workpiece along each street, the strength of the workpiece is lowered along each street. Accordingly, by applying an external force to the workpiece along each street, the workpiece is divided along each street (see Japanese Patent No. 3408805, for example).
However, a platelike workpiece such as a wafer has undulations, causing variations in surface height of the workpiece. Accordingly, in order to divide the workpiece into chips with good quality by applying an external force in each of the laser processing methods mentioned above, it is necessary to uniformly form the laser processed groove or the modified layer by detecting the surface height of the workpiece and correcting the focal position of the laser beam according to the result of detection in performing laser processing. For example, there has been proposed a detecting apparatus for detecting the surface height of a workpiece by using white light (see Japanese Patent Laid-Open No. 2008-170366, for example). This detecting apparatus utilizes the fact that the white light passed through a chromatic aberration lens has different focal lengths according to the wavelengths of the white light. Accordingly, by determining the wavelength of reflected light passed through a pinhole, the focal length of the reflected light is obtained to thereby measure the surface height of the workpiece.